Castings made of alloys in which aluminum is the primary metal, for example, present serious quality control problems because the castings produced within a production run vary substantially in physical properties. Tensile properties may vary between casting zones in the same casting due to, for example, differences in local rates of solidification. Natural variations may also occur between castings due to slight changes within the acceptable limits for adding of the constituents to form the alloy and the latitudes of time and temperature of heat treatment and processing.
Present practice in the casting art, for example, aluminum alloys, is not capable of economically mass producing castings of statistically controllable or predictable properties. Furthermore, the tests of which one skilled in the art can usually select for determining mechanical properties such as percent elongation or ductility, yield strength, or tensile strength, tend to weaken or destroy the casting.
In more recent prior art, mechanical properties of castings have been ascertained by nondestructive procedures wherein test bars poured from the same metal and at the same time as the actual casting were destructively tested and by means of coupon test bars which are poured as an integral part of the casting and broken off at the time of completion of any heat treatment of the casting. In the latter procedure the coupon test bars are also destructively tested. However, both the former and the latter procedure, exemplified by U.S. Pat. No. 3,496,766, provide a mere differential determination of the structural properties of the casting. Both methods are therefore incapable of providing reliable information of the properties of the alloy in a particular zone which may have responded to heat treatment differently from another zone.
U.S. Pat. No. 4,381,666, of which the present inventor was a coinventor, provided a method based on the discovery of a unique relationship which correlates a microstructural parameter of a cellular alloy sample to ductility. The method includes counting substantially all of the metal cells of the primary metal within a surface area of a selected zone and correlating the number of metal cells per unit area to the ductility of the zone. The number of cells is correlated to the ductility of the cellular alloy by means of the equation: ##EQU1## where EL=total average elongation (ductility) in percent,
N=number of cells of the primary metal of the alloy counted per unit area, PA1 A, B, C, D=empirical constants. PA1 U.S. Pat. No. 3,086,391, Schmitt-Thomas et al PA1 U.S. Pat. No. 3,586,546, Averbach et al PA1 U.S. Pat. No. 3,940,976, Fastner PA1 U.S. Pat. No. 4,063,644, Hoffman et al
Other known prior art include the following
Other publications
Use of Covariograms For Dendrite Arm Spacing, by R. Alberny, J. Serra, and M. Turpin;
Dendrite Cell Size, by R. E. Spear, Rsch. Engr. and G. R. Gardner, Asst. Chief, Castings and Forgings Div., Alcoa Research Laboratories, Cleveland;
Quality Control Practices, by Emmett N. Bossing and John J. Hall The Relation of Ductility To Dendrite Cell Size In A Cast, Al-Si-Mg Alloy, by S. F. Frederick and W. A. Bailey;
LEITZ-T.A.S. Texture Analysing System, Ernst Leitz GmbH Wetzlar and Robert Bosch Fernsehanlagen GmbH, Darmstadt.